It is conventional practice in radiation therapy to cast beam-forming field shaping blocks specifically individualized to each patient. The eutectic alloy field shaping blocks are usually cast in a polyfoam block such as polystyrene which has been especially cut to make the mold. One method of cutting the field shaping block molds is to place the patient x-ray a set distance from a pivot which simulates the radiation source and to place a block of rigid polyfoam a set distance between the x-ray and the pivot and to cut the polyfoam with a hot wire cutter which follows either a pivoted rigid rod or a long wire cable as the operator traces the outline of the designated treatment areas on the x-ray.
The voids thus created in the polyfoam are then filled with a molten eutectic alloy which cools to become one or more field shaping blocks. The blocks are then placed on a treatment tray in the beam of the radiation therapy machine, such as an accelerator or cobalt unit. The field shaping blocks are manipulated into position on the treatment tray to shield the areas of the patient which are to be protected from radiation.
While this current practice of defining the radiation beam is certainly better than the older practice of simply arranging rectangular lead blocks on the treatment tray, the whole process leaves much to be desired, and it leads to many cumulative inaccuracies because the process described above cannot orient field shaping blocks with sufficient accuracy for use with modern high-powered, well-collimated treatment machines. Even slight errors in orientation can seriously undermine the physician's intent by allowing some areas to go untreated while others are seriously over-exposed. Furthermore, in casting the blocks, conventional cooling trays do not provide uniform pressure in holding the polyfoam block, and they permit spillage to occur.
Accordingly, it is an object of this invention to provide an improved system of indexing and orienting accurately formed cast field shaping blocks in radiotherapy treatment apparatus. It is another object to provide a cooling tray for cast field shaping blocks.